Yet another way:

$ cat master        
key1 other 1 stuff
key2 other 2 stuff
key3 other 3 stuff

$ cat subset 
key1 more 1 stuff
key3 more 3 stuff

$ cat ex.pl 
#!/usr/bin/perl -w
use strict;
my %hash;
while(<>){
    chomp;
    my($key, $rest) = split " ",$_,2;
    $hash{$key} = $rest and next if 1 .. eof;
    $hash{$key} .= " $rest" if exists $hash{$key};
}
for (sort keys %hash){
    print "$_ $hash{$_}\n";
}
__END__

$ perl ex.pl subset master 
key1 more 1 stuff other 1 stuff
key3 more 3 stuff other 3 stuff
[download]